Electroluminescence is the energy absorbed by molecules that are subject to electric current or a strong electric field that elevates the electrons to the excited state, which can be created by various sources. The most widely used and convenient source is ultraviolet (wavelength from 100 to 400 nm) or visible light photons, which can be simply accomplished even from a hand-held UV lamp.
The efficiency of electroluminescent devices based on organic fluorescent materials is hardly obtained higher than 25%, because under electrical excitation, 25% of the excited photos are in a singlet state, while the others (75%) are in the triplet state. Phosphorescent material is, therefore, required for such application. However, it is expensive to prepare phosphorescent materials as they normally possess heavy metals such as Ir (III) and Pt (II) ions. Although organic materials show the advantages of structure and low price, their fluorescence properties make them less attractive for device application.
Another optical phenomenon is chemiluminescence where the light is emitted due to a chemical reaction. Luminol or 3-aminophthalhydrazide is a chemiluminescence compound that gives bright blue light when oxidized by hydrogen peroxide or blood. It is widely used in crime scene investigations of trace blood wiped by the suspect. Chemiluminescence from luminol derivatives are widely investigated, but some other types of known chemiluminescence systems, using peroxyoxalic derivatives, adamantine derivatives, coelenterazine derivatives, and/or acridinium derivatives have also been investigated.
However, these days, electroluminescence and chemiluminescence are generated with the help of UV light, which is known to be a major source of human skin cancer. For this reason, there is a need for fluorescent materials that can emit light without the help of UV excitation.
A recent phenomenon that has been discovered is triboluminescence or mechanoluminescence. Triboluminescence is an optical phenomenon where the crashing or pressing of chemical bonds generates light. This phenomenon is of particular importance because light is generated without aid of an excitation source, for example UV light. This kind of effect can be used broadly in anti-counterfeiting, disposable application or sensing the instant motion generated in short amount of time upon the material with triggering the material collapsed to give off emission.
Organic dyes are rich in variety and have been widely used as readily processable light-emitting materials, particularly in the area of organic optoelectronics. Due to their poor miscibility with water, organic dyes are prone to aggregate in aqueous media, which normally weakens their light emissions. This effect is commonly known as aggregation-caused quenching (ACQ).
For sensitive detection, fluorescent materials must emit intense visible light upon photoexcitation. However, light emissions from most luminophores are rather weak. This aggregation-caused quenching (ACQ) is due to emission quenching caused by the aggregation of luminophores in the solid state. When dispersed in aqueous media or bound to biomolecules, luminogenic molecules are inclined to aggregate, which usually quenches their fluorescence, and thus greatly limits their effectiveness as bioprobes. The ACQ effect also makes it difficult to assay low-abundance molecular species in biological systems because the fluorescence signals from minimal amounts of luminophores matching the bioanalyte levels may be too weak to be determined accurately. In addition, at high luminophore concentrations, the emissions are further weakened, rather than enhanced, due to the ACQ effect.
Accordingly, there is a great need for the development of fluorescent materials that exhibit electroluminescence, and chemiluminescence to obtain readily available light-emitting materials, particularly in the area of organic optoelectronics. Further, there is also a need to develop materials exhibiting triboluminescence properties that are able to emit light without UV excitation.